Despite the board's attempts to discourage me from attempting such a big beer with my first all grain attempt, I have decided that I'm gonna give it a shot. It's a recipe I've worked up on my own for a Belgian Strong Dark Abbey style. This is the recipe:
14 lbs. Belgian Pilsner
2 lbs. Homemade dark candi sugar
.5 lbs. Caramunich
.5 lb. Biscuit
.5 lb. Aromatic
1 lb. Special B
1 oz. Challenger @ 40
1 oz. E.K.G. @ 20
Two stage starter yeast - WLP500
Mash at 150 for 90 min. Fly sparge 7.5 gallon. Boil 90 min. Pitch at 70, and let naturally heat to 80 and keep under 80 for two weeks. Keg, purge air, and pressurize to 30 psi and age at room temp for 3 weeks.

If anyone has any suggestions on the recipe 'd love to hear them but really, I'm more curious about technique. What should I expect, what should I be on the lookout for? What's the worst I could go wrong. I'm not too concerned with missing my target SG of 1.090. I figure a similar profile in a smaller beer would be a perfectly enjoyable result, but what might I do to keep it up?

You may also want to mash longer that 60 with that much grain. I would think a 90 minute mash at 150.... that was my problem with my last big beer, mashed to high for too short a time .... did not get the numbers I was looking for...

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" I figure a similar profile in a smaller beer would be a perfectly enjoyable result, but what might I do to keep it up?"

Practice on some smaller beers first? Sorry - couldn't resist....

To maximize efficiency on a big brew, start with a very thin mash - ie more water per lb of grain. Stir the heck out of it when doughing in. Let it mash a bit longer than usual. Calculate your sparge water so that you get a gallon more than your recipe calls for. eg if your target starting boil volume is 7 gallons, collect a total of 8 gallons. Let it boil down in your brewpot until you hit 7 gallons and then start your normal 60 minute boil.

Have you fermented a big beer before? If not, you'll want to look into some of those procedures too.

The "worst" that can happen is you end up with a sickenly sweet brew from missing your mash temps and not fermenting fully.

@TheZymurgist - On the contrary: I said I cannot be discouraged. I can listen quite well. I am a bit confused as to lower mashing temp. The information I've read often uses the term "produces a dryer beer". Is that to say that is produces a more fermentable wort? In the case of a Belgian Strong a highly fermentable wort would be preferable, but a dry beer would not. I was hoping someone would touch on this, because many of the recipes I've looked through call for higher mash temps... but I thought lower would make more sense.

@BrewerinBR - Is there a point of ill effect, or very diminishing returns as far as increased mash time go, or is it better to go as long as I can hold temp?

@Bill - Yes, I've fermented large beers successfully. I intend to make a stir plate to enhance my starter for this beer.

The grain bills of Belgian ales lead to a very malty sweet beer. Sugar additions and low mash temperatures are traditionally used to dry the beer out and make it more balanced and more "digestible." A "dry finish" is highly desirable in Belgians - even more so in Trappist examples of the styles.

A lot of the perceived sweetness comes from the yeast character, so a highly fermentable wort (and thus, "dry" beer) can still seem sweet, even though the residual sugars are quite low compared to other styles.

Most commercial Belgians are mashed low (or step mash), so if that's what you're trying to emulate, I would mash similarly.

A lot of the perceived sweetness comes from the yeast character, so a highly fermentable wort (and thus, "dry" beer) can still seem sweet, even though the residual sugars are quite low compared to other styles.

Most commercial Belgians are mashed low (or step mash), so if that's what you're trying to emulate, I would mash similarly.

I agree.
The Trappist yeasts are aggressive and produce lots of pleasant esters that can sometimes be perceived as sweet. (I normally ferment these at 75F and hold temp) The specialty malts and the caramelized sugars in your syrup will contribute to additional sweetness. Trappist beers range from moderately dry to very dry (Triple). Mash low (148-150) and long to convert the majority of the pilsner. You want your FG to end up under 1.012 to keep it from turning out too sweet; it helps if you add the syrup to the primary a few days into fermentation. This can be challenging but your hard work will be rewarded! Good Luck.

Longer mash time (if you can hold the temp) will help the conversion of starch to sugars and increase efficiency so you get the most of the grain. Large grain volume longer mash time becasue there is more to convert..

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I seek not a hobby to fill my time; I seek a passion to fill my life!